This invention relates to the use of a thin film evaporator to purify N,N-dialkylcarbamoylmethyl phosphonates and phosphine oxides. In particular, it relates to the use of a thin film evaporator to purify dihexyl N,N-diethylcarbamoylmethylphosphonate (CMP) and octylphenyl N,N-diisobutylcarbamoyphosphine oxide (CMPO).
The use of monodentate and bidentate organophosphorus ligands as extractants for actinides and lanthanides is well-known in the extractant art. Typical monodentate ligands include tributyl phosphate and dibutylbutyl phosphonate. These monodentate ligands are effective for extracting uranium, neptunium, and plutonium from typical nitric acid solutions. However, the monodentate ligands are incapable of extracting americium and curium from nitric acid solutions due to extensive co-extraction of nitric acid into the organic phase, resulting in back-salting.
Unlike the monodentate ligands, bidentate organophosphorus ligands are superior for extracting actinides and lanthanides, and are thus useful for removing lanthanides and actinides from liquid wastes generated by processing or reprocessing nuclear fuels and from weapons production. In particular, CMP has been used in processes designed to separate the biologically hazardous radioactive actinide elements with long half-lives from the bulk of process wastes. For instance, a 30% solution of CMP in various solvents is effective for removing greater than 99% of the americium and plutonium from the waste streams at a plutonium reclamation facility. However, the lack of availability of bidentate extractants such as CMP in high purity has hampered the commercialization of such processes.
It is known that dialkyl N,N-dialkylcarbamoylmethyl-phosphonates can be prepared in 40% to 60% yield by the Arbuzov reaction of trialkyl phosphites with N,N-diethyl bromoacetamides. But the quality of the product produced from this reaction is generally poor due to the decomposition of the product and to accompanying side reactions.
In the Michaelis reaction a dialkylphosphite can also be used: ##STR1## That reaction also suffers from a low yield due to the slow dissolution of sodium.
U.S. Pat. No. 4,396,556, issued Aug. 2, 1983, to Kem, uses a variation of the Michaelis reaction employing aqueous sodium hydroxide as the base and a phase-transfer catalyst such as methyl tricaprylylammonium chloride to avoid hydrolysis of the reactants and products. The process of this reference provides for the preparation of carbamoylmethylphosphonates using phase transfer catalysis under reaction conditions that avoid the degradative hydrolysis of products and reactants.
Although various processes are effective in producing bidentate organophosphorus extractants in good yield, the final products frequently include amounts of impurities which exceed the specifications required for their use in critical applications such as the removal of radioactive actinides from liquid wastes. The present invention is directed to the purification of such extractants to meet these standards.